Plunger assembly and writing instrument



Jan. 28, 1969 A. DOTTLINGER PLUNGER ASSEMBLY AND WRITING INSTRUMENT Sheet Filed Dec. 19, 1966 Jan. 28, 1969 A. DOTTLINGER PLUNGER ASSEMBLY AND WRITING INSTRUMENT Filed Dec. 19, 1966 Sheet ',2 of 2 United States Patent A 11,471/ 65 U.S. Cl. 401--112 Int. Cl. B43k 7/12, /16, 24/06 13 Claims ABSTRACT OF THE DISCLOSURE A plunger assembly, which has special utility in writing instruments, such as ball-point pens, having an extensible and retractible writing tip carrier consists of a plunger and a guide sleeve At least one of the parts is made of elastic material to permit radial movement between the parts. At least one of said parts has a buttress thread, which cooperates with the other of said parts. Said buttress thread has a trailing flank which has an angle of lead which is so small that said trailing flank is self-locking when it engages said other part. The buttress thread has also a pressure face, which has an angle of lead which is larger than any angle of lead that would render said pressure face self-locking when engaging said other part. The self locking action prevents rotation of the plunger in the sleeve during forward movement of the plunger. When the plunger is released, a retracting spring causes the plunger to rotate to a forward or rear locked position along the pressure flank of the buttress thread.

This invention relates to a writing instrument, particularly a ball-point pen, which comprises a plunger mechanism, which consists only of a guide sleeve and a springloaded plunger, which is mounted for rotational and guided longitudinal movement and by a mere pressure on its end face is caused to snap alternately into a forward notch and a rear one. Writing instruments having such a mechanism which consists only of two parts have the advantage that the writing tip carrier can be pushed into and locked in its writing position and unlocked and retracted to itsinoperative position without complicated manipulations, and that only two parts must be manufactured for the plunger assembly. As these two parts, namely, the plunger and the guide sleeve, are anyway required in a pressure-actuated ball-point pen, it is sufficient to give these parts a proper shape so that they can easily be assembled in a telescopic arrangement. This assembly can be performed fully automatically and at low cost. In all two-part plunger assemblies, the detent elements are integral with the plunger and the guide sleeve, respectively, the plunger being integral with a detent element and the guide sleeve being integral with another detent element.

It is an object of the invention to eliminate disadvantages inherent in prior art writing instruments and provide a writing instrument which has a minimum number of parts and comprises a plunger assembly which consists suitably of injection-molded plastics material and can easily be assembled and ensures a perfectly reliable function even under the action of a substantially inclined and eccentric force whereas the assembly affords a perfect freedom as to the design of the plunger and enables the same to be provided with a perfectly smooth end face.

The writing instrument according to the invention is mainly characterized in that the plunger and/ or the guide sleeve are radially resilient relative to each other and provided on their periphery or a portion of their periphery with buttress threads, which have such a lead that the trailing flanks of the threads have an inclination within the self-locking range and the pressure flanks of the threads have an inclination outside said range.

Owing to the wedge action of the trailing flank of the thread, the self-locking action prevents a rotation of the plunger in the threads of the guide sleeve during the forward movement of the plunger. The same wedge action resiliently forces the plunger and the guide sleeve apart in a radial direction until the pressure flank of the buttress thread of the plunger snaps over the pressure flank of the female screw thread of the guide sleeve. When the plunger has been released by the finger, the retracting spring causes the same to rotationally slide into the forward or rear locked position along the pressure flanks of the buttress threads, which pressure flanks have an inclination outside the self-locking range. This locked position is determined by the respective end of the screw threads, their lead, and their axial offset. When the plunger had been depressed from its rear locked position, it will rotationally return to the forward locked position, in which the writing tip carrier is in writing position. When the plunger is depressed from this position, it will be subsequently returned to its initial position, in which the writing tip carrier is inoperative. As the snapping of the detent element into the locked position is aided in this case by a flexural rather than by a rotational spring action because the male screw thread offthe plunger snaps radially into the female screw thread of the guide sleeve, and the self-locking action of the trailing flanks of the buttress threads prevents an action of a reaction torque on the plunger, it is not necessary to all to provide the plunger with a rough end face. There is a perfect freedom in the design of this end face and of the plunger in general.

When the male screw thread on the plunger snaps radially into the female screw thread on 'the guide sleeve, the pressure flanks of both screw threads overlap according to the invention in such a large length that this overlap is preserved in part even in the case of a disturbing rotation of the plunger during its forward stroke. This fact ensures the fulfilment of the most important requirement for a reliable function of a two-part plunger assembly in which a detent element is fixed to the plunger or the guide sleeve. To increasethis reliability, the guide sleeve and/or plunger is provided with an abutment rib for engagement with the top end of the screw thread of the plunger, and/or the plunger is provided with an abutment rib for engagement with the lower end of the female screw thread of the guide sleeve. These abutment ribs serve also for guiding the respective screw threads during the forward movement of the plunger and positively prevent a disturbing rotation of the plunger toward this rib under the influence of an eccentric and inclined force.

To ensure that even an opposite disturbing rotation away from the abutment and guide rib under the action of an accentric and inclined force will not distunb the function, it is a further feature of the invention that the abutment and guide rib is inclined to the longitudinal axis of the writing instrument in the same direction as the screw thread. This design anticipates the disturbing rotation of the plunger from the abutment and guide rib and ensures a positive guidance of the plunger and an undisturbed function of the plunger assembly under the action of any eccentric or inclined force, whether this force acts on the plunger on the left or right of its center. It is a further feature of the invention that the plunger or the guide sleeve is provided on its periphery or a portion of its periphery with buttress threads, which are closely spaced in the longitudinal direction, the lead of the screw threads is a multiple of their pitch, and the ends of the two screw threads on the same part are approximately on the same radial plane. This design facilitates the axial Sliding movement of the plunger, a sufficiently large projection of the writing tip carrier from the forward end of the body of the writing instrument, and a high strength of the plunger assembly under axial load, even Where plastics material is used, as the 'axial force is taken up mainly by the overlapping pressure flank portions of the buttress thread of the plunger and of the guide sleeve. These pressure flank portions resiliently snap one over the other during the locking and releasing operations. The subsequent screwing of one thread into the other under the action of the retracting spring further increases the resistance to axial load. The resistance to axial load will be still further increased if, in a further development of the invention, the pressure flanks of the buttress threads of the plunger and of the guide sleeve are at an angle of about 70 75 relative to the longitudinal axis of the writing instrument.

In order to enable an leastic deformation of the guide sleeve or plunger in a radial direction, a sufliciently large radial clearance is required for the guide sleeve in the body of the writing instrument or for the plunger, more particularly its buttress threads, in the guide sleeve. When the top portion of the body of the wriitng instrument is used as a guide sleeve for the plunger, the cross-section of the latter should be weakened or shaped in such a manner that it can be gently deflected during its forward movement. In this case the plunger has suitably an enlarged portion near its lower end and this enlarged portion has a funnel shaped cavity for centrically receiving the upper end of the writing tip carrier. To improve the guidance of the plunger, the outside diameter of said enlarged portion corresponds approximately to the inside diameter of the adjacent portion of the body. The funnel itself is suitably shaped in such a manner that it adds to the flexural compliance of the plunger. With plunger assemblies which are relatively large in diameter, e.g., when the upper portion of the body of the writing instrument is used as a guide sleeve, it may be desirable to provide all elements of the assembly in each of two sets, which are spaced 180 apart on the periphery of the plunger and guide sleeve, respectively, and to form the plunger with a longitudinal slot, which before the snapping of the buttress threads of the plunger into the mating female threads of the guide sleeve permits the threads of the plunger to yield radially inwardly on both sides.

Several embodiments of the invention are shown diagrammatically and by way of example on the drawing, in which 7 FIG. 1 is an elevation showing a guide sleeve in the form of a clip-retaining screw,

FIG. 2 is an elevation showing the associated plunger having two buttress threads, the upper ends of which are on the same radial plane,

FIG. 3 shows partly in section the plunger assembly consisting of a guide sleeve of FIG. 1 and a plunger of FIG. 2 in assembled condition and screw into the upper part of the body of the writing instrument with the writing tip carrier in retracted or inoperative position,

FIG. 4 is an elevation showing the same assembly with the plunger and writing tip carrier in writing position,

FIG. 5 is an elevation showing the same assembly with a plunger which has been fully depressed beyond the writing position,

FIG. 6 is an elevation, partly in section, showing a guide sleeve which has two female buttress threads and is screwed into the upper part of the instrument body, which is injection-molded with an integral clip, the ends of the butress threads of the guide sleeve being on the same radial plane, 1

FIG. 7 is an elevation showing a corresponding plunger having a single buttress thread,

FIG. 8 is an elevation of a guide sleeve which is formed by the upper portion of an instrument body and intended to cooperate with a plunger having a single buttress thread,

FIG. 9 is an elevation showing the plunger for cooperation with this guide sleeve,

FIG. 10 is an elevation showing a modification of the plunger guide of FIG. 8, and

FIG. 11 is an elevation showing the plunger for cooperation with the guide of FIG. 10.

A plunger 3 is slida'bly and rotationally guided in the upper end portion 1 of the body of the writing instrument or in a clip-retaining screw 2 or guide sleeve 4 (FIG. 6) screwed into such upper portion. The plunger acts on the writing tip carrier 5, which is loaded by a retracting spring. As shown in FIGS. 1-5, the detent means on the plunger comprise two buttress threads 6 and 7, which extend around a portion of the periphery of the plunger. Under the action of the retracting spring, the pressure flank 23 of the buttress thread 6 can slide back along the pressure flank 9 of the guide sleeve or clip-retaining screw 2 until the triangular end face 10 of the second buttress thread 7 engages the abutment rib 11 of the guide sleeve, and/ or the abutment rib 12 of the plunger 3 engages the triangular end face 24 of the mating female buttress thread 22 of the guide sleeve or clip-retaining screw 2 when the rear or initial position of the plunger 3 and the inoperative position of the writing tip carrier, FIG. 3, has been reached. When the plunger 3 is being depressed toward the clip-retaining screw 2, as is shown in FIG. 3, the plunger will initially move freely in an axial direction in its guide along the abutment rib 11 until the wedge-shaped trailing flank 13 of the second buttress thread 7 engages the corresponding trailing flank 14 of the female buttress thread in the clip-retaining screw 2. The trailing flanks 13 and 14 have preferably the same inclination. Owing to the self-locking eflect of the wedge-shaped trailing flanks 13 and 14 of the buttress screw threads 7 and 22 of the plunger and guide sleeve, respectively, and the elastic radial deformation of the clip-retaining screw 2 and/or the plunger 3, the lower buttress thread 7 snaps over the pressure flank 9 of the female buttress thread 22 in the lower portion of said pressure flank 9 and the pressure flank 23 of the thread 7 then slides back along the pressure flank 9 until the triangular end face 15 of the upper buttress thread 6 of the plunger engages the lower end of the abutment rib 13 and/ or the triangular abutment face 25 of the plunger 3 engages the triangular end face 24 of the female buttress thread 22 of the clip-retaining screw 2 so that the writing tip carrier is locked in writing position, as shown in FIG. 4. When the plunger 3 is being depressed once more, as is shown in FIG. 5, the upper buttress thread 6 of the plunger 3 becomes effective. When this thread 6 has snapped over the lower portion of the pressure flank 9 of the female buttress thread 22 of the clip-retaining screw 2, the thread 6 will return along the flank 9 to the initial position, shown in FIG. 3.

According to FIGS. 15, two buttress threads 6, 7 on the plunger and a single female buttress thread 22 are used. FIGS. 7-11 show plungers having only one buttress thread 16 and cooperating guide sleeves having two female buttress threads 26 and 27. FIGS. 6, 8 and 10 show an abutment rib 19 which is inclined relative to the longitudinal direction of the writing instrument in the same direction as the respective thread 16 (FIGS. 7, 9, and 11), and serves also as a guiding rib during the forward movement of the plunger. In FIG. 6, the plunger 3 has one buttress thread 16. When this plunger is depressed from its rear position, its triangular end face 20 slides forwardly along the abutment rib 19 until the wedge-shaped trailing flank 13 of the thread 16 engages the mating trailing flank 14 of the female thread 26 of the guide sleeve 4 (FIG. 6). When the buttress thread 16 has resiliently snapped over the female thread 26 and the plunger 3 is subsequently released, the pressure flank 23 of the buttress thread 16 will slide along the pressure flank 18 of the female thread 26 until the triangular end face 20 of the thread 16 engages the triangular abutment 21 of the female thread 26 and/or the abutment rib 28 of the plunger engages the also triangular end face 29 of the female thread 27 of the guide sleeve. The writing tip carrier is now in writing position. When the plunger is fully depressed once more, the same operation is repeated, with the difference that the pressure flank 23 of the buttress thread 16 of the plunger resiliently snaps over the lower portion of the pressure flank which is engageable with the pressure flank along this pressure flank to its initial position.

In the assemblies shown in FIGS. 1 to 6, the guide sleeve 4 or the clip-retaining screw 2 is elastic in a radial direction. The assemblies shown in FIGS. 8-11 operate with a flexural elastic deformation of the plunger. For this purpose, the plunger may be so weakened or shaped in its cross-section that it can be gently deflected and easily actuated.

What is claimed is:

1. A plunger assembly consisting of two parts, namely, a guide sleeve and a plunger which is axially and rotationally movably mounted in said guide sleeve, at least one of said parts being elastically movable relative to the other in a radial direction, at least one of said parts having a buttress thread, which has a trailing flank engageable with the other of said parts and having an angle of lead which is sufliciently small to render said trailing flank self-locking relative to the other of said parts when in engagement therewith, and a pressure flank engageable with the other of said parts and having an angle of lead which is larger than any angle of lead which would render said pressure flank self-locking relative to the other of said parts when in engagement therewith.

2. A plunger assembly as set forth in claim 1, in which said buttress thread extends only on a portion of that peripheral surface on said one part which faces the other of said parts.

3. A plunger assembly as set forth in claim 1, in which said plunger has a male buttress thread and said guide sleeve has a female buttress thread, each of said buttress threads has a trailing flank which is engageable with the trailing flank of the other buttress thread and has an angle of lead which is sufficiently small to render said trailing flanks self-locking when they engage each other, and a pressure flank which is engageable with the pressure flank of the other buttress thread and has an angle of lead which is larger than any angle of lead which would render said pressure flanks self-locking when they engage each other.

4. A plunger assembly as set forth in claim 3, in which said male buttress thread is adapted to snap over said female buttress thread so that said pressure faces of said buttress threads overlap for a distance which is so large that said overlap is preserved at least in part even when the plunger is subjected during its axial movement to a torque which opposes that dervied by said buttress threads.

5. A plunger assembly as set forth in claim 3, in which said plunger comprises an abutment rib which is engageable with one end of said female buttress thread, said guide sleeve comprises an abutment rib which is engageable with one end of said male buttress thread which is opposite to said one end of said female buttress thread, and said abutment ribs are adapted to guide the respective thread ends during the axial and rotational movement of said plunger.

6. A plunger assembly as set forth in claim 3, in which each of said parts is provided with two of said buttress threads, which are spaced 180 apart, and said plunger is provided with a longitudinal slot between its two buttress threads so that the latter can yield radially inwardly and snap over said buttress threads of said sleeve.

7. A plunger assembly as set forth in claim 1, in which the other of said parts comprises an abutment rib, which is engageable with one end of said buttress thread and adapted to guide said end during the axial and rotational movement of said plunger.

8. A plunger as set forth in claim 7, in which said abutment rib is inclined relative to the longitudinal axis of said other part in the same direction as said angle of lead of said buttress thread.

9. A plunger assembly as set forth in claim 1, in which the other of said parts is provided with two buttress threads which are spaced apart in the longitudinal direction of said other parts, the lead of each of said buttress threads is a multiple of its pitch, and each of said buttress threads of both said parts has a trailing flan'k which is engageable with any trailing flank of such buttress thread of the respective other part and has an angle of lead which is sufficiently small to render said trailing flanks self-locking when they engage each other, and a pressure flank which is engageable with any pressure flank of such buttress thread of the respective other part and has an angle of lead which is larger than any angle of lead which would render said pressure flanks self-locking when then engage each other, the ends of said two buttress threads on said other parts lying substantially in a radial plane with respect to the longitudinal axis of said other part.

10. A plunger assembly as set forth in claim 1, in which said pressure flanks are at an angle of 70-75 relative to the longitudinal axis of said one part.

11. A plunger assembly as set forth in claim 1, in which said guide sleeve is formed by an upper portion of a body of a writing instrument and the plunger has a cross-sectional shape so that it is gently deflectable during its axial movement and has an enlarged lower portion formed with a funnel-shaped cavity, said enlarged portion has an outside diameter which corresponds approximately to the inside diameter of the adjacent portion of said guide sleeve, and said cavity is designed to increase the flexural compliance of said plunger.

12. A plunger assembly as set forth in claim 11, in Which said plunger is weakened in its cross-section to increase its flexural compliance.

13. A writing instrument which comprises a body, a writing tip carrier which is contained in and longitudinally movable relative to said body between an extended writing position and a retracted position, means urging said Writing tip carrier toward its retracted position, and a plunger assembly carried by said body and arranged to control the position of said writing tip carrier, said plunger assembly consisting of two parts, namely, a guide sleeve and a plunger which is axially and rotationally movably mounted in said guide sleeve, at least one of said parts being elastically movable relative to the other in a radial direction, at least one of said parts having a buttress thread, which has a trailing flank engageable with the other of said parts and having an angle of lead which is sufliciently small to render said trailing flank self-locking relative to the other of said parts when in engagement therewith, and a pressure flank engageable with the other of said parts and having an angle of lead which is larger than any angle of lead which would render said pressure flank self-locking relative to the other of said parts when in engagement therewith,

References Cited UNITED STATES PATENTS 3,179,087 4/1965 Kahn et al 401-112 LAWRENCE CHARLES, Primary Examiner. 

